rebar3 plugin entry point for erli18n catalog tooling.

init/1 is the single hook rebar3 calls when it loads the plugin. It chains the five providers' own init/1 functions, each of which registers its provider under the erli18n namespace, so that after load the project has:

• rebar3 erli18n extract — walk abstract forms into .pot templates;
• rebar3 erli18n merge — msgmerge-style .po sync;
• rebar3 erli18n check — CI gate that fails on .pot drift;
• rebar3 erli18n report — per-(Domain, Locale) completeness;
• rebar3 erli18n compile — opt-in compile-time .po->BEAM catalog codegen.

The plugin is a SEPARATE Hex package from the runtime erli18n library and depends on it ({deps,[{erli18n,"~> 0.7"}]}, {applications,[...,erli18n]}), pointing plugin -> lib — the same direction as rebar3_gpb_plugin -> gpb. The providers reuse the published PO read/serialize API (erli18n_po:parse/1, erli18n_po:dump/1, erli18n_po:escape_string/1) across that boundary. Consumers opt in with {plugins,[rebar3_erli18n]} in their own rebar.config.

Compile-time .po->BEAM codegen is OPT-IN: the compile provider is a loud-logged no-op unless {compiled_catalogs, true} is set in rebar.config, so a project that uses none of the compiled catalogs sees zero change. When enabled it bakes each catalog's ALREADY-parsed entries plus its ALREADY-compiled Plural-Forms rule into a generated carrier (see rebar3_erli18n_prv_compile and rebar3_erli18n_codegen), which erli18n:register_compiled_catalogs/1 installs at boot with no runtime parse or plural compile.